1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus which is capable of stably and efficiently conveying a developer along an inclined conveying pipe.
2. Discussion of the Background
Background image forming apparatuses, such as printers, facsimiles, copiers, and multifunction apparatuses which print, fax, copy, and so on generally use an electrophotographic process for image forming. Such background image forming apparatus includes a development unit which employs a trickle development method to supply a new carrier.
Using the trickle development method, the new carrier is supplied by a necessary amount to the development unit, which stores a two-component developer including toner and the carrier. The toner is supplied from a toner compensating port into the development unit by the necessary amount in accordance with the consumption of the toner in the development unit. The toner compensating port is arranged at a portion of the development unit.
The supplied toner is stirred and mixed with the developer in the development unit by stirring members such as a conveying screw and so on. After the developer is stirred and mixed, a part of the developer is then supplied to a development roller. The developer held on the development roller is controlled to be a necessary amount by a doctor blade. The toner, which is one component of the two-component developer, is adhered on a latent image formed on a photosensitive drum at a position at which the photosensitive drum faces the development roller.
In a common development process, the carrier, which is another component of the two-component developer, is not consumed and remains in the development unit. Therefore, the carrier deteriorates in time. More specifically, a charging ability of the carrier may be decreased because a coating layer of the carrier is worn off or is peeled off while the carrier has been stirred or mixed during repeated processes for a long period (film peeling phenomenon). Further, the charging ability of the carrier may be decreased because toner elements and additives are adhered onto the surface of the carrier (spent phenomenon).
The trickle development method is employed to avoid the image degradation due to such carrier deterioration in time. Namely, the new carrier (or new two-component developer) is supplied when it is necessary and a part of the new two-component developer in the development unit is output from the development unit as appropriate. The charging ability of the carrier is then maintained and the amount of the carrier stored in the development unit is maintained constant by reducing the portion of the deteriorated carrier in the development unit.
The image forming apparatus using the trickle development method can produce a high quality image for a longer usage comparing to an image forming apparatus in which the development unit and the carrier are exchanged with the new development unit and the new carrier only when the degradation of the carrier is detected.
An image forming apparatus includes a coil screw in a conveying path from a carrier storage to the development unit. More specifically, the coil screw is arranged inside a hollow pipe connecting a carrier source to a carrier target arranged separately from each other. The carrier is conveyed by a mechanical force generated by a drive rotation of the coil screw. Further, the carrier may be conveyed by a screw pump through a carrier conveying path formed with a tube having a flexible shape.
In such image forming apparatus, the carrier may slip through an interspace between the hollow pipe and the coil screw. The conveying amount of the carrier may be small, especially in the case where the conveying path is inclined and the carrier is conveyed from a lower position to a higher position against the gravitational force.
Even when the image forming apparatus uses an-air pump to convey the carrier through the inclined path against gravity, a conveying failure may still occur because a specific gravity of the carrier is larger than a specific gravity of the toner. As a result, a strong and large pump requiring a high power may be needed.